(1) Field of the Invention
The present invention is directed to a two parameter clutter map which provides control of false alarms from clutter interference in a radar receiver by storing estimates of the clutter characteristics in each resolution cell. The maps store two measured parameters of clutter at each spatial location to define its mean echo strength and scan-to-scan fluctuation characteristics. In addition, alarms on leading edges of moving rain storms can also be controlled.
(2) Description of Related Art
The prior art radars, particularly moving target detector types, employ a fine-grain clutter map having cells less than a beamwidth times a pulse width to estimate the mean echo voltage for each spatial location. The detection threshold is the sum in decibels (dB) of the map value and an operator selectable margin.
In order to obtain an accurate estimate of the mean average, many scans are necessary. Inaccurate estimates demand a loss in sensitivity in order to maintain a constant false alarm rate (CFAR). In addition, a target on a tangential flight path can impact a given map cell for many scans causing an apparent azimuth shift in the detected target. This leaves a trail of lower sensitivity behind each target. In order to minimize the above effects a sluggish map response is desirable.
In contrast, rain storms create changing interference conditions at a given map location. To prevent excessive alarms caused by the leading edges of the storms, a fairly rapid map response is desired. This presents a conflict with the sluggish map response necessary to obtain an accurate estimate of the mean. A satisfactory compromise is not available since long range radars have long scan periods.
Another limitation associated with this type of clutter map is that it presumes that the scan-to-scan fluctuation characteristics of all varieties of clutter are identical to the fluctuation characteristics of noise. In reality, terrain clutter echoes are generally considered to have fluctuating (ac) and non-fluctuating (dc) components. The ac to dc ratio depends on the extent of foliage cover and wind velocity. The dc component represents not only immovable objects but also the moving foliage when movement is a small fraction of a wavelength.
In moving target detectors, multiple echoes received as the beam scans across a target are coherently integrated in a bank of Doppler filters. In the Doppler filters responding to zero Doppler, the fluctuation characteristics of noise and clutter are preserved since there is no post-detection integration. Because noise fluctuations have no dc component, their fluctuations are worse than terrain clutter, and the map works well to control alarms from the terrain clutter. Although sensitivity is reduced more than necessary in areas where clutter fluctuations are less extreme, this is a tolerable sacrifice.
In radars which employ post-detection integration, pulse-to-pulse fluctuations are attenuated by the integration process. The wider the spectrum of interference relative to the radar pulse repetition frequency, the greater the attenuation of the input fluctuation by the integration process. Noise fluctuations are attenuated most and terrain clutter is attenuated least. Using this type of clutter map (e.g., estimating mean interference voltage or power) sacrifices most of the sensitivity benefit of post-detection integration in noisy areas because the detection threshold has to be set far above that necessary for noise in order to tolerate the more severe scan-to-scan fluctuations from clutter. The increase in transmitter power for this type of map is generally unacceptable.
The present invention solves the above problems, allowing post-detection integration to be employed for improving sensitivity of the radar without sacrificing false alarm control.